A well known method of forming thermoplastic polymers involves placing a sheet of resin over a male tool, placing a vacuum impervious layer or bag over the sheet and tool, sealing the edges of the bag, heating the thermoplastic to a temperature above its glass transition temperature, drawing a vacuum through the tool or vacuum ports to pull the sheet onto the tool, and cooling the assembly to fix the shape of the formed thermoplastic sheet.
In order to fully form a sheet by this method, it is necessary that a vacuum can be pulled continuously through the bag on both the tool and bag sides of the sheet to be formed. In the past, this has been accomplished by pulling a vacuum through small orifices in the tool and/or interposing one or more layers of fiberglass between the bag or tool and the sheet to be molded. An example of such a method is shown in U.S. Pat. No. 3,703,422 which describes the manufacture of honeycomb panels by gluing polyimide face sheets onto honeycomb core. A breather layer of glass or metal mesh, coated with a non-stick release agent, is provided above and below the laminate. U.S. Pat. No. 3,810,815 shows interleafing a metal mesh strip between layers of polyacrylic and polycarbonate sheet to make clear laminated windows for aircraft. The sheet becomes embedded in the laminated panels and is cut off as scrap.
This invention relates particularly to bonding fiber-reinforced, high temperature thermoplastic preforms by a vacuum bagging method to achieve structural panels without the use of adhesives. We found that prior art methods of vacuum bagging high temperature thermoplastics were unsuccessful because the breather layer would deteriorate at the temperatures needed to fuse preforms and layups of reinforced thermoplastic. In particular, coatings on the breathers would themselves fuse and prevent uniform vacuum over the entire panel during the forming process and cool down.